RE: [net-next,PATCH v2] enetc: Increase eMDIO MDC rate to 2.5 MHz

[Wei Fang <wei.fang@nxp.com>]

Sorry, this email was missed because it did not match the filtering rules I set.

Hi Valdimir and Claudiu,

Please take a look at this patch, it may affect LS1028A and S32N7 platforms.

> The current eMDIO MDC rate is 558 kHz on i.MX95 B0. This is very low,

The system clock of i.MX95 NETC is 333MHz, AFAIU, the MDC rate should be
333/(2*258+1) = 644KHz. I'm confused that why you said the rate is 558KHz.

> but also very compatible. However, for example NXP TJA1120 PHY needs
> at minimum 1 MHz MDC clock and 558 kHz does not work with that PHY.

We previously used the TJA1120-SDBR daughter card to connect to the ENETC
interface (RGMII) of the i.MX95 19x19 EVK board. However, we did not encounter
any issues with MDIO access. Furthermore, I did not see a minimum MDC
frequency rate of 1MHz on the TJA1120 datasheet. I'm not sure if this is an errata
of TJA1120.

> 
> As a minimum change, increase the MDC clock to 2.5 MHz which matches

The prerequisite is that NETC system clock is 333MHz, so that you can get a
MDC clock rate of 2.5MHz (333/(2*66 + 1)). But unfortunately, not all NETC
versions have a 333MHz system clock. For example, LS1028A is 400MHz, so
the MDC rate will be 3MHz, which will be a problem.

> the 802.3 specification. Also, even old PHYs like LAN8210Ai, KSZ8794,
> DP83822 support up to 2.5 MHz MDC clock frequency, and it covers the
> NXP TJA1120 as well. There is no known rationale for why the 558 kHz
> was picked when the eMDIO driver was added.
> 

> Signed-off-by: Marek Vasut <marex@nabladev.com>
> ---
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Andrew Lunn <andrew+netdev@lunn.ch>
> Cc: Clark Wang <xiaoning.wang@nxp.com>
> Cc: Claudiu Manoil <claudiu.manoil@nxp.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
> Cc: Wei Fang <wei.fang@nxp.com>
> Cc: imx@lists.linux.dev
> Cc: linux-kernel@vger.kernel.org
> Cc: netdev@vger.kernel.org
> ---
> V2: Use 2.5 MHz per 802.3 spec, update commit message
> ---
>  drivers/net/ethernet/freescale/enetc/enetc_mdio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/freescale/enetc/enetc_mdio.c
> b/drivers/net/ethernet/freescale/enetc/enetc_mdio.c
> index 998aaa394e9c4..a90a04a47befd 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc_mdio.c
> +++ b/drivers/net/ethernet/freescale/enetc/enetc_mdio.c
> @@ -24,7 +24,7 @@
> 
>  #define ENETC_EMDIO_CFG \
>  	(MDIO_CFG_HOLD(2) | \
> -	 MDIO_CFG_CLKDIV(258) | \
> +	 MDIO_CFG_CLKDIV(66) | \
>  	 MDIO_CFG_NEG)
> 
>  #define MDIO_CTL_DEV_ADDR(x)	((x) & 0x1f)
> --
> 2.53.0